Gardenias require acidic soil, direct sunlight or bright reflected light, moist(not soggy) soil, excellent drainage and high humidity. Temperature is also critical, especially at flowering time, as high temperatures(above 85°F) can cause bud drop and few flowers. (They are winter flowering plants in their Asian sub-tropical habitat).
It is often difficult to meet all of these requirements, when they are grown as potted house plants, plus you will probably have to deal with insect infestation as well.
Move them outdoors as soon as last chance of frost has passed, gradually acclimate them to direct sunlight(early morning or very late afternoon, at first), be sure to check moisture frequently and fertilize with Miracid or similar type.
In other words, treat them like a Rhododendron or Azalea!
I have grown them for more than 25 years(potted and in-ground) and the roots even survived temperatures of -12°F, in the ground.
Rb

Thank you Rb. I have the pot outdoors on the patio and, because I live in a condominium, I'll have to leave it there. But now I'm confused because the instructions that came with it said "morning sun only" and said it's a shade plant.

I guess the high humidity is the reason I could grow them in Florida. Do you think I could mist the plant as a kind of substitute?

There's some interesting reading on gardenias here:

Here is a link that might be useful: infamous suicidal gardinia thread

you can set the pot on a bed of gravel in a large overssized soucer and fill the soucer with water to raise humidity around the plant. My experience with gardenias is they do rather poorly if in full sun all day. seems they do better treated like a camillia. I think the instructions of morning sun is probably a good direction to follow. Also consider the specific variety you have. Plants are cultivated in different varieties for there specific characteristics. It would be a big help to research info on that variety. But as a rule of thumb everything RB mentioned is correct. When I moved hear from the coast, about 170 miles inland, I took a cutting from one of my gardenias and rooted it. I planted it in full sun all day and it has only grown to 2 1/2 feet and rarely blooms. I plan to relocate it this fall. Working on a new bed for it this summer. There is an azalea in the bed next to it and doing fine. So the soil is probably suitable, just too much sun. I also wonder what effects the indoor temps have on your plant when you bring it in for the winter. The plant may be getting confused what season it is so to speak. Maybe you have a garage to put it in rather than a warm house. Preferably with glass windows on the garage door to get some light in. also you might could build a mini green house to slip over it for winter and leave it outside.

Karchita, I'm so glad someone saved that old gardenia thread. It was so funny. (There was another very funny one--dogs in elk, or something like that--remember?)

I have no comment on gardenias except we just yanked two out and put in daylilies.